This project investigates four hypotheses concerning the specificity of tyrosine kinases. Constitutive or inappropriate activation of tyrosine kinases plays a role in the progression and development of various forms of human cancer. The proposed research focuses on the insulin-like growth factor I receptor (IGF1R) and Jak2 tyrosine kinases, two enzymes that have been implicated in malignant transformation. The overall hypothesis for this work is that tyrosine kinase specificity is regulated at two levels: (1) by the spatial/temporal organization of the kinase signaling pathway; and (2) by the specificity intrinsic to the kinase catalytic domain. Aims 1 and 2 compare the signaling properties of the IGF1R and insulin receptor (IR) tyrosine kinases. The first hypothesis is that the differences in specificity between these two closely related receptors can be explained by differences in the structural and enzymatic properties of the catalytic domain. Aims 1 and 2 compare the signaling properties of the IGF1R and insulin receptor (IR) tyrosine kinases. The first hypothesis is that the differences in specificity between these two closely related receptors can be explained by differences in the structural and enzymatic properties of the catalytic domains. The proposed experiments will test this hypothesis by analyzing IGF1R autophosphorylation, activation, and substrate specificity between these two closely related receptors can be explained by differences in the structural and enzymatic properties of the catalytic domains. The proposed experiments will test this hypothesis by analyzing IGF1R autophosphorylation, activation, and substrate specificity. The second hypothesis to be investigated is that differential phosphorylation of IRS-1 and other cellular proteins is involved in specific signaling by IR and IGF1R. Aims 3 and 4 investigate signal transduction by the Jak2 non-receptor tyrosine kinase. The propose studies will test the hypothesis that this enzyme is regulated by autoinhibition and by intermolecular autophosphorylation. Additional experiments will address the importance of a specific interaction between Jak2 and the serine/threonine phosphatase PP2A during cytokine signalling. The goal of these studies is to provide new information on the molecular basis of substrate specificity for IGF1R and Jak2. A systematic understanding of tyrosine kinase substrate specificity is important both for identifying physiological effectors, and for designing strategies to interrupt specific signalling pathways.